The invention relates to a colour display tube comprising an envelope consisting of a neck, a cone and a display window, and in the neck having an integrated electron gun system of the "in-line" type to generate three electron beams situated substantially with their axes in one plane. The electron gun system comprises successively a cathode, a grid and an anode to generate each electron beam and at least two electrodes for focusing each electron beam on the display screen which is provided on the display window. The electrodes, which are common to the three electron beams, produce an electric field which forms focusing lenses for the electron beams when a suitable potential difference is applied. The electrodes each include a cup-shaped part, the bottom of which has three apertures through which the electron beams pass.
Such a colour display tube is known from Netherlands Patent Application No. 7904114 laid open to public inspection, corresponding to U.S. Pat. No. 4,337,409. The three electron beams can be focused to form spots on the display screen by focusing lenses of the bipotential type consisting of two electrodes which are at different potentials. However, it is also possible to use focusing lenses of the unipotential type consisting of three electrodes, the first and the last electrode of which have the same potentials. It is also possible to focus with four successive electrodes which can be electrically interconnected in different manners. An embodiment of this latter type is described in Applicants' Netherlands Patent Application No. 8102526 not yet laid open to public inspection, corresponding to U.S. patent application Ser. No. 370,430 filed Apr. 21, 1982. The electrodes of all the above-described lens types in integrated electron gun systems each consist of at least one cup-shaped part the bottom of which has three apertures through which the electron beams pass. A collar is usually provided around each of the apertures and extends into the cup-shaped part, usually parallel to the cup wall.
The disadvantage of such so far used electron gun systems is that often as a result of astigmatism in the focused electron beams parts of the displayed picture are not sharp. This astigmatism occurs substantially due to the lack of competely circular symmetry of the apertures and collars. The circular symmetry is lacking because of defects occurring during the manufacture of the electrodes. During the manufacture and assembly of the gun components, errors may also be introduced which cause the location of the three spots of the three electron beams on the display screen to be incorrect.